The world-wide shortage of 3He gave rise to urgent needs for highly sensitive neutron detectors having neutron/gamma discrimination similar to 3He detectors. The applications included safeguarding nuclear materials and weapons, treaty verification, anti-proliferation, and the recovery of lost military payloads. More recently, however, the desire to guard against nuclear smuggling, the potential use of a radiological weapon (so called “dirty” bombs), and other terrorist acts, has given rise to needs to perform neutron surveillance at border and port facilities, transportation systems and other places where large amounts of a cargo or people pass by or through on a regular basis. Such neutron surveillance must be accomplished without undue restriction or disruption of traffic flow and events.
One class of conventional neutron detector is the gas-filled counter, typically based on helium-3 gas contained in high pressure (around 2 bar) tubes. Despite the fact that the helium-3 filled drift tubes are sensitive to microphonics, high-pressure helium-3 drift tubes have the best overall performance compared with other methods; that is, thermal neutron efficiencies above 80% and excellent neutron/gamma discrimination. These types of conventional neutron detectors are effective and therefore are the preferred choice in many types of operations, including oil logging operations, cryogenics for low temperature physics research, and medical applications such as diagnosis of chronic obstructive pulmonary diseases. However, the supply of helium-3 is limited, and therefore, large scale deployment of helium-3 is not an option. Thus, alternatives to helium-3 based neutron detection are necessary to meet the needs of portal monitoring and other increasing demands.
Another class of conventional neutron detectors is scintillation-based detectors, which is based on photon-emitting transitions that occur in the wake of energetic charged nuclei released from collisions between incident neutrons and atomic nuclei. Scintillation devices include a transparent neutron sensitive material (either a gas or a liquid or solid) that generates light upon receipt of incident neutrons. The scintillation devices are typically coupled to a photomultiplier tube to generate an analog electrical signal based on the production of the light within the scintillation material. The analog signal is indicative of the incident neutron irradiation. To enhance the efficiencies of the scintillators, the neutron sensitive materials are typically doped with lithium-6 and boron-10. However, neutron/gamma ray discrimination remains an issue for scintillators, and must be resolved in order for scintillators to becoming practical for helium-3 replacement.
Yet another class of neutron detectors includes solid state neutron detection devices based on thin films of boron-10 or lithium-6 coated onto silicon and other substrates. These devices usually use enriched 10boron or lithium-6 for highest efficiency possible. However, the highest theoretical efficiency for a single layer is limited to only a few percent. Meanwhile, charge losses in the substrate also limit the ultimate efficiency for multi-layer approaches.